1. Field of the Invention
The present invention relates to an amplifier, more particularly to a fully differential amplifier.
2. Description of the Related Art
A fully differential amplifier has an enhanced immunity to external noise, and therefore, is often used in modern analog integrated circuits. Since the common mode loop gain of fully differential amplifiers is typically insufficient, drifting in a common mode level of differential output voltages may occur. Hence, a common mode feedback (CMFB) circuit is required to establish the common mode level.
Referring to FIG. 1, a conventional fully differential amplifier comprises a first amplifier stage 1, a second amplifier stage 2, and a CMFB circuit 3. The CMFB circuit 3 includes a common mode level detector 31 and an amplifier 32. Since the fully differential amplifier is a closed-loop system, it encounters stability problems. If frequency compensation is inadequately performed, oscillation of the amplifier will occur. There are the following two conventional types of common mode frequency compensation methods:
(1) Reduce Loop Gain
In a state where pole frequency is unchanging, the unity gain frequency may move down in frequency when the loop gain becomes small, such that a phase margin becomes large. In the conventional configuration, a pair of resistors (not shown) may be included in the amplifier 32 to reduce loop gain using source degeneration. Alternatively, a pair of current sources (not shown) may be included in the first amplifier stage 1 to reduce loop gain by minimizing a current ratio of a feedback current mirror. However, such methods may cause a reduction in bandwidth, an increase in offset voltage, and an increase in common mode noise.
(2) Increase Zero
In addition to the above methods, it is possible to include in the CMFB circuit 3 a zero generator (not shown) A phase lag of a pole is reduced using a phase lead generated by adding zeros, such that a phase margin is increased. However, this method is significantly influenced by process drift, and the zero generator requires large resistors and capacitors, such that zeros added thereby are in a low frequency range. Also, it is necessary to provide a large physical area for placement of the zero generator due to the use of such large resistors and capacitors.